Active control of noise

ABSTRACT

The control of noise emanating from engines and their exhaust systems is a difficult problem to overcome in a cost effective way. The noise may be reduced by the introduction of a fluid into the exhaust system such that the fluid is heated and expands rapidly producing sound and vibration which is in antiphase to the noise and vibration produced by the engine acting through the exhaust. There is consequent reduction in the sound and vibration produced by the system. The fluid introduced is conveniently a liquid (water) which enhances the damping effect by virtue of possessing a latent heat of vaporisation. The fluid can be added to the exhaust system by a bleed from a turbo charger or super charger or by alternate means. A primary use would be for the reduction of engine noise emanating from exhaust systems in heavy goods vehicles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for the activecontrol of noise in exhaust systems.

2. Discussion of Prior Art

Conventional exhaust systems consist of a length of pipe with silencersconnected to an engine. The effect is to dampen the sound and vibrationproduced by the exhaust gases.

As there is no perfect exhaust system there is a requirement to improvethe level of sound or vibration reduction which can be achieved.

Consequently the present invention seeks to reduce the sound andvibration in an exhaust system by a method of active noise control.

SUMMARY OF THE INVENTION

Accordingly there is provided a method for reducing the noise producedby an engine's exhaust system which comprises the introduction of extrafluid to the exhaust system in addition to that produced by the exhaustoutput of the engine characterized in that the introduction of fluidinto the system is suitably timed such that the noise source (exhaust)and noise produced by the fluid introduction are of different phase andideally opposite.

The term exhaust system is applicable to any hot gas flow where noise ispresent.

It is thought that by the suitably timed addition of a fluid such as airor water into an exhaust system that the sound and vibration which thisproduces can be in antiphase to the originating exhaust sound andvibration. This results in a reduction or cancelling of the sound andvibration. The total noise output of the system will actually beincreased if the noise produced by fluid injection is in phase with theoriginal exhaust noise.

To produce sound an effluent gas must contract or expand to create avolume velocity, that is, there must be a net volume change. Thus volumechanges in the exhaust lead to noise.

By adding a fluid to the exhaust there is an initial increase in volumewhich may be offset by cooling or other effects which result in anoverall decrease in volume. These volume changes can oppose thoseoccurring naturally in the exhaust. The effects of volume increase andcontraction cooling can be experienced with various fluids but additionof liquid such as water has the further effect of vaporisation thusresulting in a volume increase. The latent heat of vaporisation for aliquid introduced to the exhaust has the effect of cooling the pressurepeaks. Therefore if cold water is added to the hot gases of an exhaustsystem the effects of heat extraction and vaporisation compete. In thiscase the cold water and subsequent vaporisation cool the gas causing acontraction while water vapor produced by the vaporisation causesexpansion.

Preferably the fluid introduction takes place in the proximity of theoutlet manifold of the exhaust.

Preferably the fluid introduced is a liquid. The boiling point of anyliquid introduced should be lower than the temperature of the exhaustgases to ensure vaporisation of the liquid.

Preferably the fluid introduced is water.

Whilst there are various ways of introducing the fluid it is preferablyinjected into the exhaust system. In the case where turbo chargers orsuper chargers are fitted the fluid may be pressurized into the exhaustvia a bleed from the turbo charger or super charger. Alternatively, fuelinjectors can be used for fluid introduction to the exhaust system.

Preferably the fluid, when a liquid is introduced into the exhaustsystem, is a spray.

The effective operation of the invention's active noise control dependsto some extent on adopting the appropriate procedure for a particularnoise source. For example where there are large source strengths it isnecessary to have low exhaust temperatures together with a fine spray ofliquid, preferably water, injected into the exhaust to achieve goodactive noise control.

Alternatively for noise sources of high bandwidth, high Mach numbers forexhaust velocity and high exhaust temperatures should be used togetherwith a fine spray of liquid and turbulent mixing of liquid and exhaustgases.

Bandwidth of the active noise reduction system can be increased byensuring that Forced convection occurs for the lifetime of the spraydroplets. This may be achieved by disturbing the Flow up and downstreamof the injector or other entry system to the exhaust.

Furthermore sound level and hence active noise control may be increasedby ensuring that the Final temperature is less than 100° C. It isbelieved that this is because the latent heat has been partly extractedwithout vaporisation. Final temperature as used herein refers to thattemperature at which there is no net change in injected fluid volumewithin the exhaust which yields noise.

The invention is considered to have particular utility in reducing thenoise levels in exhaust systems of heavy goods vehicles.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only and withreference to the accompanying Drawings in which FIG. 1 shows a graph ofvolume change For water against initial temperature and where Δ is thevolume change factor produced by injection of X moles of water (at 15°C.) per mole or exhaust gas at temperature T_(a1) ; and FIG. 2 is ablock diagram of the present invention.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

For sound production it is necessary to have a volume change in a gas.Exhaust gases from an engine expand as they move from a high pressurearea to a low pressure area. A sound absorbing system should thus beeffective to produce volume contraction in the exhaust gas flow. Whenwater is injected into a hot exhaust system there are four states of theisobaric process to consider. These are the initial state uponinjection; the mixing, heat exchange and expansion; vaporisation; andequilibrium where the final temperature is greater than 100° C.

    ______________________________________                                        State                                                                         ______________________________________                                        1.  n.sub.a moles of hot gas, volume V.sub.a1,                                                         Gas     H.sub.2 O liquid                                 Temperature T.sub.a1, and                                                                          V.sub.a1                                                                              n.sub.L, T.sub.L1                                n.sub.L moles of water at T.sub.L1                                                                 T.sub.a1, n.sub.a                                    2.  Allow mixing, heat exchange                                                                        Gas: V.sub.a2, T.sub.a2                                  and expansion        H.sub.2 O: T.sub.L2 = 100° C.                 3.  Vaporisation         Gas: V.sub.a3, T.sub.a3                                                       H.sub.2 O vapour:                                                             T.sub.v2 = 100° C., n.sub.v = n.sub.L         4.  Equilibrium at T.sub.f >100° C.                                                             Gas + H.sub.2 O:                                                              V.sub.f, T.sub.F, n = n.sub.a + n.sub.L              ______________________________________                                    

The heat required to vaporize n₁ moles of water under constant pressureequals the enthalpy extracted from the exhaust gas (states 1 and 3):

    H.sub.v =n.sub.L (c.sub.L c.sub.pL (T.sub.L2 -T.sub.L1))=--c.sub.pa n.sub.a (T.sub.a3 -T.sub.a1)

where

c_(L) =the latent heat of vaporisation of water

c_(pL) =specific heat capacity of water at constant pressure

c_(pa) =specific heat capacity of gas at constant pressure

Rearrangement gives

    T.sub.a3 =T.sub.a1 -X(c.sub.L +c.sub.pL (T.sub.L2 -T.sub.L1))/c.sub.pa =T.sub.a1 -BX

where X=n_(L) /n_(a) is the ratio of water to gas in the mix.

For the change from vaporisation to equilibrium. (state 3 to state 4)assuming perfect gas behavior (valid for small X) then if there is noheat exchange with the outside environment dH=0 and H₃ =H_(f), that is:

    c.sub.pa n.sub.a (T.sub.a3 -T.sub.0)+c.sub.pv n.sub.v (T.sub.v3 -T.sub.o)=c.sub.p n(T.sub.f -T.sub.o)

where T_(o) =a reference temperature

For mixtures of perfect gases some thermodynamic properties of themixture are just the weighted sum of the corresponding properties of theconstituents (Gibbs-Dalton Law). This is applied here: ##EQU1##

Applied above and rearranged gives:

    T.sub.f =RT.sub.a3 +T.sub.v3 /1+R

where R=(1/X)(c_(pa/c) _(pv))

Finally, for a perfect gas the fractional volume change from thetemperature change is:

    Δ=V.sub.f V.sub.a1 =(1+X)T.sub.f /T.sub.a1

The water must remain as vapor in the final state for this expression tobe valid. A petrol engine exhaust contains typically 75% N₂, 10% CO₂ and15% H₂ O assuming stoichometric combustion. The specific heat capacityof the exhaust gas will be dominated by that of Nitrogen. By taking theinitial water as T_(L1) =15° C. and molar heat capacities c_(pa) =34Jmol^(-1K-1), c_(pv) =36 Jmol^(-1K-1), c_(pL) =75.4 Jmol^(-1K-1), c_(L)=40600 Jmol⁻¹ (these values applying at atmospheric pressure and at therelevant temperatures), gives a set of curves of as a function of X andinitial exhaust temperature T_(a1), as shown in FIG. 1. The figure showsthat, for water at least, there is always a net contraction in volume.

Where the invention has been applied to a 60 kw diesel generator by theinjection of water into the exhaust system using a relatively simplearrangement an acoustic source was produced of comparable strength tothe engine at frequencies below 25 Hz. Such a system is illustrated inFIG. 2 where the engine 20 includes an exhaust system 22. Spray liquidinjector 26, responsive to the phase of the exhaust sound injects liquidinto said exhaust out of phase or antiphase to the originating exhaustsound. In a preferred embodiment, the exhaust system 22 includes anexhaust manifold 28 which joins an exhaust pipe 30. The fluid injector26 injects a water spray near the outlet of the exhaust manifold 28. Apreferred embodiment uses the bleed from a super charger 32(alternatively a turbo charger 34) to pressurize a liquid supply 36which provides liquid to liquid injector 26.

We claim:
 1. A method for reducing the noise produced by an engine'sexhaust system which comprises the introduction of a liquid to theexhaust system in addition to that produced by the exhaust output of theengine; wherein the introduction of liquid into the system is suitablytimed such that said noise produced by an engine's exhaust system andnoise produced by the liquid introduction are of different phase.
 2. Amethod of noise reduction as claimed in claim 1 wherein the liquidintroduction takes place in the proximity of the outlet manifold of theexhaust.
 3. A method of noise reduction as claimed in claim 1 whereinthe liquid is water and a final temperature is less than 100° C. wheresaid final temperature is that temperature at which there is no netchange in injected fluid volume within the exhaust which yields noise.4. A method of noise reduction as claimed in claim 1 wherein the fluidintroduced is water.
 5. A method of noise reduction as claimed in claim1 wherein the liquid is injected into the exhaust system.
 6. A method ofnoise reduction as claimed in claim 1 wherein turbo chargers or superchargers are fitted to pressurize liquid into the exhaust via a bleedfrom the turbo charger or super charger.
 7. A method of noise reductionas claimed in claim 1 wherein the liquid is introduced into the exhaustsystem as a spray.
 8. A method of noise reduction as claimed in claim 1wherein a fine spray of liquid is injected into the exhaust.
 9. A methodof noise reduction as claimed in claim 1 wherein, for noise sources ofhigh bandwidth, high Mach number exhaust velocities and high exhausttemperatures are used in combination with a fine spray of liquid andturbulent mixing of liquid and exhaust gases.
 10. A method of noisereduction as claimed in claim 8 wherein the bandwidth of the activenoise reduction system is increased by ensuring that forced convectionoccurs for the lifetime of the spray droplets.
 11. A method of noisereduction as claimed in claim 10 wherein forced convection is achievedby disturbing the flow up and downstream of the injector or other entrysystem to the exhaust.
 12. An active noise reduction system for reducingperiodic exhaust noise in an engine having an exhaust system, said noisereduction system comprising:phase measuring means for measuring thephase of said periodic exhaust noise; and liquid injector means,responsive to said phase measuring means, for injecting liquid into saidexhaust system antiphase with respect to said periodic exhaust noise.13. The apparatus of claim 12, wherein said liquid injector means is aspray liquid injector.
 14. The apparatus according to claim 13, whereinsaid liquid is water.
 15. The apparatus according to claim 12, whereinsaid liquid is water.
 16. The apparatus according to claim 12, whereinsaid exhaust system includes an exhaust manifold with an outlet and saidliquid injector means includes means for injecting liquid near saidoutlet.
 17. The apparatus according to claim 16, wherein said liquid iswater.
 18. The apparatus according to claim 17, wherein said injectedwater is in the form of an injected water spray.
 19. The apparatus ofclaim 12, wherein said engine includes a super charger, and said liquidinjector means includes a liquid supply, said super charger comprising ameans for pressurizing said liquid supply.
 20. The apparatus of claim12, wherein said engine includes a turbo charger, and said liquidinjector means includes a liquid supply, said turbo charger comprising ameans for pressurizing said liquid supply.